1. Field of the Invention
The present invention relates to a driving method of an operational transconductance amplifier, and more particularly, to a driving method for improving power efficiency of an operational transconductance amplifier.
2. Description of the Prior Art
Advantages of the liquid crystal display (LCD) include lighter weight, less electrical consumption, and less radiation contamination. Thus, the LCD has been widely applied to several portable information products such as notebooks, PDAs, etc. The LCD is gradually replacing the CRT monitors of conventional desktop computers. Incident light will produce different polarization or refraction when alignments of these liquid crystal molecules are different. The LCD utilizes the characteristics of the liquid crystal molecules to control the light transmittance and produce gorgeous images.
Please refer to FIG. 1. FIG. 1 is a schematic diagram of a conventional thin film transistor (TFT) liquid crystal display (LCD) monitor 10. The LCD monitor 10 comprises an LCD panel 12, a controller 14, a first driving circuit I6, a second driving circuit I8, a first voltage generator 20, and a second voltage generator 22. The LCD panel 12 comprises two substrates. An LCD layer is filled in the space between these two substrates. One substrate is disposed with a plurality of first data lines 24, a plurality of second data lines 26 which are perpendicular to the first data lines 24, and a plurality of thin film transistors 28. The other substrate is disposed with a common electrode (not shown) for providing a stable voltage (Vcom) by the first voltage generator 20. For convenience, only four thin film transistors 28 are illustrated in FIG. 1. In fact, the thin film transistors 28 are disposed on the LCD panel 12 in a matrix format. That is, each of the thin film transistors 28 is disposed on the intersection of each of the first data lines 24 and each of the second data lines 26. Each first data line 24 corresponds to a column of the LCD panel 12, each second data line 24 corresponds to a row of the LCD panel 12, and each thin film transistor 28 corresponds to a pixel. Additionally, the circuit characteristic formed by the substrates can be deemed an equivalent capacitor 30.
A driving principle for the conventional LCD monitor 10 is described as follows. When the controller 14 receives horizontal synchronization signals or vertical synchronization signals, the controller 14 provides corresponding control signals respectively to the first driving circuit I6 and to the second driving circuit I8. Then the first driving circuit I6 and the second driving circuit I8 provide input signals to the first data lines 24 and the second data lines 26 by determining the control signals. Next, the input signals received by the first data lines 24 and the second data lines 26 change the states of the thin film transistors 28 and the voltage of the equivalent capacitor 30. Finally, the alignment of the liquid crystal molecules and the light transmittance are changed. Therefore, changing the voltage of the input signals provided from the first driving circuit I6 and from the second driving circuit I8 can change the gray level of the corresponding pixel. For example, if the second driving circuit 26 transmits a pulse to the second data lines 18 to turn on the thin film transistor 28, the first driving circuit I6 can transmit signals to the equivalent capacitor 30 through the first data lines 24 and the thin film transistors 28 to control the gray level of a corresponding pixel. Additionally, the signals, transmitted from the first driving circuit I6, of the first data lines 24 are generated from the second voltage generator 22.
Please refer to FIG. 2. FIG. 2 is a schematic diagram illustrating an operational amplifier buffer (op buffer) 40 circuit of the conventional LCD monitor 10 shown in FIG. 1. The op buffer 40 is a class-A driver amplifier. The op buffer 40 is used to drive the LCD monitor 10 so that each pixel on the LCD monitor 10 can reach a predetermined gray level. When a voltage Vin turns on a transistor 41 and a bias voltage Vb turns on transistors 42, 43, a first stage circuit 44 of the op buffer 40 will drive a second stage circuit 45 of the op buffer 40 to generate a corresponding output voltage Vout with current I3. The voltage Vout is used to drive the LCD monitor 10. Because the op buffer 40 is a class-A driver amplifier, it bears a high power efficiency. That is, most power-consumption of the op buffer 40 is used to drive the LCD monitor 10. For example, the sum of currents I1, and I2 is assumed to be 10 uA and the current I3 derived from the op buffer 40 might be 100 uA. That is, the current I3 is much greater than the currents I1, and I2. In other words, most electric power consumed by the op buffer 40 is used for driving the LCD monitor 10.
Concerning a dot inversion driving applied on the LCD monitor 10, a positive driving buffer is used for pulling up voltage of a pixel from a negative polarity to a positive polarity, and a negative driving buffer is used for pushing down voltage of the pixel from the positive polarity to the negative polarity. Therefore, each of the positive driving buffer and the negative driving buffer is only responsible for driving pixels toward a positive or a negative polarity according to the dot inversion driving. The class-A operational amplifier with small bias current is generally adopted to be the required positive or negative driving buffer owing to great power efficiency on driving single polarity. Although the op buffer 40, which is a class-A operational amplifier, bears high power efficiency, yet it still needs a compensating capacitor 46 and an output resistor 47 to control the output slew rate of the op buffer 40. Thus, a bigger layout area and a higher manufacturing cost of the op buffer 40 are inevitable.
Please refer to FIG. 3. FIG. 3 is a schematic diagram illustrating a conventional operational transconductance amplifier (OTA) 50 circuit. A voltage Vin turns on a transistor 51. A bias voltage Vb turns on a transistor 52 and keeps the transistor 52 in a saturation state. Because the voltage at node D is not large enough to turn on a transistor 53 in the beginning, the transistor 53 is cut-off and current I5 equals current I4. Although the OTA 50 bears many advantages such as a smaller size, a simpler structure, and a good slew rate (no extra compensating capacitors or output resistors are necessary), yet the power efficiency of the OTA 50 is not high. As described previously, since the current I5 is equal to the current I6 before the voltage at node D is equal to the voltage Vin to turn on the transistor 53, the power efficiency of the OTA is only 50% (power efficiency=I6/(I5+I6)).
In conclusion, contrary to the op buffer 40, the OTA 50 bears advantages of smaller size and simpler structure. However, the low power efficiency for the OTA 50 prevents it from being applied to the LCD monitor 10.
It is therefore a primary objective of the claimed invention to provide an operational transconductance amplifier with simpler structure, smaller size, but higher power efficiency to solve the above-mentioned problems.
The claimed invention provides a driving method for improving power efficiency of an operational transconductance amplifier. The operational transconductance amplifier comprises a first current route and a second current route symmetrical to the first current route. Both of the first current route and the second current route comprise a plurality of transistors. Each of the transistors of the first current route has a smaller width/length ratio than the corresponding transistors of the second current route. The driving method comprises turning on the transistors of the first current route for outputting a reference current so that the second current route outputs a mirror current, which is greater than the reference current, corresponding to the reference current.
It is an advantage of the claimed invention that the operational transconductance amplifier can achieve both high power efficiency and good slew rate by only adjusting the ratio between the W/L ratio of the transistors disposed on the first current route and the W/L ratio of the transistors disposed on the second current route. Therefore, a great amount of current intensity is generated at an output terminal of the operational transconductance amplifier until the voltage level of the output terminal approaches a required value, and high power efficiency is acquired as well.
These and other objectives of the claimed invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.